1. Field of the Invention
The present invention relates to a semiconductor device, particularly for high power switching of the type known, for example, from European Patent A1-0 121 068.
2. Discussion of Background
Turn-off power semiconductor devices in the form of GTO thyristors (GTO=Gate Turn Off), FCThs (FCTh=Field Controlled Thyristor) or SIThs (SITh=Static Induction Thyristor) are becoming increasingly important in power electronics, particularly in the case of static converters or electric drive systems.
In addition to high current rating in the conduction case and high reverse voltages, the main emphasis in these devices is placed on good controlability, particularly with respect to turning them off via the control electrode or control zone.
For the construction of GTO thyristors and FCThs, to which the further discussions are restricted, a step-shaped so-called "recessed-gate" structure consisting of gate and cathode fingers alternately following each other has been proposed in the printed document initially quoted, in which structure the entire semiconductor substrate outside the remaining cathode fingers is removed except for the lower plane of the gate contact. As can be seen quite clearly in FIG. 1 of this printed document, the lower gate plane in the completed device (GTO or FCTh) forms a new continuous main plane from which the remaining cathode fingers project upwards.
Although the construction of these known devices with "recessed-gate" structure has the advantage of being very easily contacted on the cathode side, this is counteracted by restrictions which have an unfavorable effect particularly with respect to the achievable control behavior, but also during production.
On the one hand, if a trench is used in the edge area outside the control zone given by the cathode fingers in order to achieve a high reverse-voltage rating, a deep selective and thus comparatively complex p-type diffusion must be provided in an additional process step to neutralize the negative influences of the P-N junction emerging at the surface.
On the other hand, the distinctly stepped structure after the deep etching results in problems during all subsequent photolithographic processes since the photoresist distributes itself with greatly differing thickness on the highly structured substrate surface. This leads to problems particularly during the etching of the gate metallization in the gate trenches so that, in practice, only the gate bottom can be metallized so that the electrical separation between gate and cathode contact is achieved with sufficient reliability. For this reason, the trenches must keep within a particular width which, in turn, restricts the achievable thickness of the cathode fingers in the control zone.